Variable time sound recording method and system



July 27, 1948. E. w. KELLQGG 2,445,332

VARILABLE TIME soum: RECORDING METHOD AND SYSTEM Filed No v. 19, 1943 2 Sheets-Sheet 1 No/sE Esouc-nou AMPLIFIER AMPLIFIER M/XEE AMPLIFIER & fizn Aeo WkzzLoc-zs,

. INVENTOR.

ATTORN EY.

July 27, 1948.

E. w. KELLOGG 2,445,832

VARIABLE TIME SOUND RECORDING METHOD AND SYSTEM Filed Nov. 19, 1943 2 Sheets-Sheet 2 ibn Ak Fifi 224 ass,

. INVENTOR.

ATTORN EY.

Patented July 27, 1948 VARIABLE TIME SOUND RECORDING METHOD AND SYSTEM Edward W. Kellogg, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application November 19, 1943, Serial No. 510,857

This invention relates to sound recording systems and particularly to systems for the recording of photographic records.

Two standard types of commercial photographic recording systems are well-known, the variable area system producing a record having a longitudinal trace or traces along the film separating opaque and transparent areas, and the variable density system producing a record composed of a series of transverse striation of varying opacity or transparency. The usual manner of recording the variable area type of sound reccm is with a galvanometer having a light reflecting mirror which deflects a beam of light in proportion to the instantaneous values of the electrical currents being recorded. This current may be the alternating current alone or the instantaneous sum of the alternating current and a unidirectional current derived by rectification, which provides a bias for th reduction of ground noise by means of an aperture in a mask. Variable area systems generally shape the light beam between the lamp and the galvanometer mirror, the beam being reflected with the aperture imaged on a mask having a slit, the amount of light passing through the slit being dependent upon the deflection of the galvanometer mirror.

Although it is well-known that a galvanometer may be used as a modulator for producing variabl density records as shown in Dimmick Patent No. 2,095,318, another type of modulator for variable density systems is the light valve having two ribbons between which the light passes in an amount depending upon the amplitude of the signal being recorded. The ribbons are moved in opposite directions by the currents passing therethrough, one ribbon always moving in the direction of film motion, and the other ribbon moving in the direction opposite the film motion.

As pointed out in copending application, Serial No. 447,797, filed June 20, 1943, now Patent No. 2,354,295, granted July 25, 1944, the use of a galvanometer as 'a modulator has certain advantages over the light valve and vice versa, the invention disclosed in this copending application involving a system adapted to utilize th best features of both. The use of a galvanometer to vary the width of a constant intensity beam of light is shown wherein the results obtainable are the same as those obtainable with a double ribbon light valve. The system thus has the characteristic of variable time rather than variable intensity, which is the characteristic of the system of the present invention.

A variable area system utilizing a galvanometer 12 Claims. (Cl. 179-1003) 2 i as a modulator and producing two beams varying in width, is disclosed and claimed in myPatent No. 2,031,835 of February 25, 1936, such a system utilizing a beam dividing arrangement and a'plu-' rality of optical elements for producing the result desired. The above-mentioned copending application utilizes some of the principles involved in my patent and produces a variable density record based on varying time instead of varying intensity of the light beam. i I j x The present invention is an optical arrange ment for producing a variable density record, and has certain features in common with the abovementioned copending application; but has advantages as compared with the system shown the above copen-ding application.

It is Well-known that in the Variable time system of making variable density recordings, there occurs a wave shape distortion known as the ribbon velocity effect? This terminology has come into use because the most commonmethod of making variable time records is by means of light valves, consisting of metal ribbons which are electrically actuated to producea narrow aperture or slit, which is optically imaged on the film. In some of the light valves, known as single ribbon valves, the slit wasbetween a fixed and a movable edge. This produced considerable distortion whenever the velocity of the'moving side of the slit image became appreciable in comparison with the continuous velocity of the film. This distortion is found to be much reduced if the motion is shared by both edges, as in the double ribbon valve, in which case the amplitudes of themovements of the individual edges need be only half as great, and there is, moreover, a considerable measure of cancellation of the distortion which the sides individually would produce.

In addition to the standard recording ele:

ments, my invention utilizes a front surface mirror positioned between the galvanometer and objective lens, which mirror reflects and inverts a single directly impressed beam oi one movable edge to form a beam having two movable edges. In the system described herein, the two variable or movable edges are parallel to the longerv dimension of the rectangle of light commonly called the slit image, and the result impressed on a film is a variable exposure which gives a variable density record. i i i The principal object of the inventiomtherefore, is to facilitatethe recording of photographic A still further object of the invention is to pro-' vide an improved sound recording system in which a single variable edge beam is reflected to form two adjacent beams which increase and decrease in width along two edges.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended lai the manner of its Organization and the mode 'ofit's operation will .be better understood by referring to the following description read in conjunction with the accompanying drawings forming a part hereoflin which:

Fig. 1 is a combination block and diagrammatic view of a, film sound recording system embodying the n ent o Fig. 2 is a detailed view-of the optical portion of the system illustrating the operation thereof.

Fig. 3 is a detailed view of the light image as impressed upon the film.

Fig.4 is a diagram showing a portion of the paths of light rays, at two positions of the modulatonand.

Figure 5 is a detail View of a modification of the noise reduction feature of the invention.

Referring now to Fig. 1, signal currents generated in a microphone 5 are amplified by an amplifier 6, The amplitudes of the amplified currentsare controlled by a mixer 8 and again amplified by an amplifi r 9, he mpl fi 9 feedin a noise redu m fier In. A p on f the outpu of mplifi 9 is impressed up n a mo ulating oil H of a galvan-ometer av a mirror 3. The output of the noise reduction amplifier i is impressed on a second coil in of the galvanometer for vibrating the mirror It in accordance with the amplitude variations of the si nal currents. As an alternative method of obtaining the benefit of ground noise reduction, the biasing coil l and the conductors leading thereto may be omitted; and the conductors 33, andactuating motor 34 shown in Figure 5 substituted. The motor 35 produces vertical movementsof themask IS'in accordance with the amplitude of the audio signals, and this produces movements of the edge image In and of the images thereof on the film at b and b, similar to the movements which are produced with mask I9 stationary and the galvanometer mirror graduallly'tilted by a bias current through coil iii.

The optical portion of the recording system includes a light source 11, light from which is collected by a lens l8 and impressed upon a mask l9- havingan aperture 20 therein. 'The light emerging from the aperture 2a is reflected by the mirror I3 and brought to focus by lens 25 in the plane of a mask'21 having window 28 therein. The light emerging through the window 223 is projected to the film 30 by an objective lens iii. A front surfaced reflecting mirror 32, the operation of which willbe described hereinafter, is shown intermediate the mask 2'! and projection lens 3|. Except for the mirror 32, the above- 4 described recording system would function as a variable time image in which only a single side is variable. However, such a single side variable time system has the disadvantage already mentioned of introducing distortion known as the velocity effect.

To illustrate. the manner in which the present invention produces an image on the film, the outer edges of which move in opposite directions in accordance with the movement of the light beam by the galvanometer mirror It, reference is made to Figs. 2 and 3 wherein an end view of the lower edge of the window 28 is shown at the point a, while the upper edge be of the light beam passing through window 28 is an image of the edge In in the mask it produced by the cylindrical lens 25. Above the edge image line b2 no light passes through window 28, while the area between a and 122 is fully illuminated, being an image of the illuminated aperture 28. As the mirror rotates about its axis 22 (which in Fig. 2 is normal to the paper), the postion of b2 moves up and down, and it is obvious that with motion of be (which is the upper boundary of the illuminated area in the window 28) the illuminated area between a and Z22 will be alternately widened and narrowed.

Although only the edges a and be take part in modulating the light reaching the film, both of the mask plates l9 and 21 have apertures 28 and 28 of limited area to exclude all but the necessary light and thus reduce stray light.

The vertical axis cylindrical lens i l plays no part in the imaging of the horizontal edge in. Its function is to collect the light diverging in a horizontal direction from the mirror and direct it into the objective lens 3i. Its focal len th should preferably be such that the mirror l5 and objective El are in conjugate focal planes.

The system thus far described, with the mirror 32 omitted, would produce a rectangular image on the film, having one stationary and one movable side. The mirror 32 serves to double the image and give a symmetrical rectangle on the film, in'much the manner that reflections from a body of water give a symmetrical picture of objects on the distant shore, by adding. an inverted image, immediately under the directly viewed objects. If the mirror makesa very acute angle with the line of sight, the reflection is substantially as bright as the directly viewed object, and there need be no visible line of separation between the two portions of the image on the film. The objective lens, with the film, constitute a camera, which takes a photograph of the illuminated rectangle a-bz and its reflection. The reflection as viewed from len 35, appears to ex tend from a downward to b'z, the position of b2 being the same distance below the reflecting surface of mirror 32, as be is above it, both points being on a common normal to the reflecting surface. l

With the elements i9 and I3 positioned as shown in Fig. 2, an image such as shown in Fig. 3 will be projected on the film. If the mirror It is rotated on its axis, or, if as part of a ground noise system the mask I9 is moved, or if both of these changes take place, the image 112 will shift, and thereby cause the images b and b on the film to shift, resulting in a rectangular image of different width but always symmetrical with respect to the fixed position in, which is the image on the film of the corner a of mirror 32 and in Fig. 2, lies on the optical axis 0, while images of the point or edge 211 will be formed as shown at b and b in Figs; 2 and 3. i

It will be noted that the reflecting surfaceof:

the element 32 is positioned at such an angle that it just fails to obstruct a ray of light passing from the point a to the bottom of the objective lens 31. At this angle, light from the point or edge in at the window 28 of the mask 21 will form an image at iron the film 30by light collected by the objective lens 3!. directly from the point or line In as shown by the solid ray lines, while light from the same point or edge In at the window 28 will form an image at point 12' on the film by light reflected by the mirror 32 to the objective 3| as shown by the dotted ray lines. When the galvanometer mirror l3 rotates clockwise which, in efiect, is the equivalent of moving the element H! to the left, the point he at the window 28 will move downwardly and the two edges 5 and b of the images will approach each other, narrowing the light image on the fllm. Thus, as the mirror vibrates in accordance with either, only the instantaneous values of the signal current, or with both the instantaneous and envelope values thereof, the constant intensity light image will vary in width, thereby providing a variable time exposing system.

In order that the entire rectangle of exposed area shall be illuminatedto full brightness, the lens 25 mustconverge light through the image at 122 throughout a suiflciently large angle so that rays will pass from b; to all parts of objective lens 31, and in addition, that rays reflected from mirror 32 shall also pass to allparts of lens 3|, as indicated by the dotted lines from 112 to the mirror surface and up to the lens 3|. All portions of the window 28, between edge a and the limiting line D2, are likewise illuminated in the manner just described, so that the entire image area on the film between I) and b is equally illuminated. To completely fill the objective lens 3!, as just tiescribed, the mirror 3| must be wide enough to include the two extreme rays illustrated in Fig. 2 asradiating from in toward the lens 25, and requirements are also imposed in regard to the strength of lens 25 and the size of the pencil of light rays reaching lens 25 from In, or from any other point within the illuminated area of window 20. Whatever has been said about rays from b1, also applies to rays from all points between in and the optical axis 0, for this entire area is illuminated.

Asmentioned'above, noise reduction may be secured in two ways; one is by mechanically positioning the galvanometer so that at zero or low signal, the edge In of mask image I 9 is imaged by lens 25 closely adjacent the edge a of window 28, and then impressing the output of the noise reduction amplifier on the galvanometer winding I 5 to move the reflected image of mask [9 upwardly in accordance with the volume of the i signal currents. The other manner of obtaining noise reduction is to move the mask l9 in accordance with the volume variation of the signals which will accomplish the same result as far as the image impressed on the film is concerned.

Although the values given below for certain elements of the optical system just described will produce practical and satisfactory variable time effects, it is to be understood that other values may also be used in accordance with the size of the image desired or the method of recording employed. In the design selected for illustration, the reduction ratio of the objective lens 3| is 7.5 to 1, and it is desired to modulate the image width on the film between zero and 1 mil. The

image between a. and be at the window 28: will mirror is in its uppermost position. The ray :11

comes from the center of the mirror. The rays 1/1 and 112 001116 from in and are the outside rays of the pencil of light, as limited by the width of the galvanometer mirror. The rays marked at, an. and x2, represent the same pencil of light at a different time, when the mirror has been tipped downwardly. The axial rays 0: and y both come from the center of the mirror and both pass through the conjugate focal point 0. Thus rotation of the mirror does not cause any motion at 0, but it is obvious that at points to the left of 0, there will be motion. The plane of window 28 is located just sufliciently to the left of o to give the required 3.75 mils of movement. If the galvanometer rotates sufliciently to move the light beam .050 inch at lens 25, then the required 3.75 mils amplitude would occur at a point mils to the left of the point of zero motion, or .4625 inch from the lens. In order that light from In shall come to focus in the plane of aperture plate 21, the aperture plate I9 must be 2.94 inches from lens 25 or .94 inch back of the mirror, the distances 2.94 inches and .4625 inch being conjugate for a, lens of .4 inch focal length. Thus, the plate I9 would be placed .94 inch from the mirror.

Assuming the objective lens 3! to be .3 inch in diameter and 5 inches from th mask 21, light must converge on In throughout an an le of about 10. The lens 3| subtends about 3.5 from the aperture- 28 as calculated by the formula .-15 2 tan 1 and this angle must be doubled to supply the light which is reflected from mirror 32 and some margin added, whence we arrive at the figure 10. Since lens 25 is substantially .5 inch from b2, the light beam where it strikes the lens 25 must be .5 inch (2 tan 5) .087 inch wide. Since the mirror is about one-third of the way from In to lens 25, the actually required mirror width would be only about one-third of this or approximately .030 inch. However. for ease of adjustment, a wider mirror than this is preferable.

It is important that the light intensity be uniform at the window 23 which is best accomplished by locating the aperture 20 close to the condensing lens l8. Thelamp'filament should be imaged on the galvanometer mirror and the cylindrical lens 25 should be corrected for minimum spherical aberration, and it should be reasonably free from chromatic aberration. light from getting into the lens barrel, bafiles may be provided. It is seen that lens 25, in addition to converging light, directs all of the rays downwardly, or, in other words, it functions as a combination cylinderand prism. It is obvious that the prism function can bemade unnecessary by locating the galvanometer l3 an appropriatedis- To prevent stray design of lens 25.

7 tance' above the axis a--a1. some advantage in reducing the amount of spherical .and chromatic correction called for in the The mirror 32 should be of high quality with respect to flatness and must have a sharp edge at a, and in view of the acute angle of reflection, the amount of absorption obtained will result in very little inequality of the upper and lower halves of the image bab'. An internally reflecting prism could be used in place of the front surface mirror, but in that case, any slight rounding or chipping of the edge at a would cause a black spot or line on the image on the film whereas, with the front surface mirror, such an effect is less likely.

It would obviously be possible to use a spherical lens in place of one of the two cylinders 2d and 25, but the power required in the two planes is very different, and the use of a spherical lens would not result in any particular simplification in construction. The use of cylinders was chosen to make the explanation simpler. I-Iowever, spherical lenses may be used in a, well-known manner to permit changes in the spacings of the elements, which may be desired to meet various design considerations.

I claim as my invention:

l. A sound recording system comprising a source of light rays, means for defining said rays into a single beam, means for vibrating said beam in accordance with the amplitude of a signal to be recorded, means for directly projecting said beam to form an image thereof, said image having a fixed edge and a movable edge, and means for reflecting said beam to form an image thereof adjacent the image formed by said direct beam, said reflected image having a fixed edge and a movable edge, the vibration of said single beam vibrating the movable edges of said images and thus increasing and decreasing the width of said images.

2. A sound recording system in accordance with claim 1 in which said reflecting means has a light intercepting edge for determining the fixed edge of said images, the variation in width of said combined images being produced by the outer edges of said images moving toward and away from each other in equal amounts.

3. A sound recording system in accordance with claim 1 in which means are provided for also vibrating said beam in accordance with the variations of the average value of the amplitude of said signal.

4. A sound recording system comprising a source of light, means for forming a single elongated beam having at least one straight edge, means for moving said beam in accordance with the variations in amplitude of a signal to be recorded, and an optical system for directly forming an image or said beam on a film, said image having a fixed edge and a movable edge, said optical system comprising means for reflecting a portion of said beam to form a second image adjacent said first image, said second image having a fixed edge immediately adjacent the fixed edge of said first image, and a movable edge varying in the opposite direction to the movements of the movable edge of said first image.

5. A sound recording system comprising a source of light, means for forming an elongated beam having at least one straight edge, means for moving said beam in accordance with the variations in amplitude of a signal to be recorded, and an optical system for directly forming an image This would have of said beam on a film, said image having a fixed edge and a movable edge said optical system com-- tance between the images of said straight edge be-,

ing increased and decreased in accordance wit the movement of said beam.

6, A sound recording system in accordance with claim 4 in which means are provided'for varying the position of said straight edge in accordance with the average value of the amplitude of said signal.

7. A variable time sound recording system comprising means for obtaining a single beam of light, means for modulating said beam with signal currents to be recorded, means for directly. projecting certain rays of said beam to a certain portion of a film to form an image having a fixed edge, an optical element for varying the position of one edge of said beam during modulation of said beam, and means for simultaneously reflecting and projecting certain other rays forming a beam having a fixed edge coinciding with the fixededge of said directly projected rays and a movable edge spaced from said fixed edge substantially the same distance as said first movable edge is spaced from said first fixed edge.

8. A sound recording optical system comprising a single light beam defining element, an objective for forming an imag oflight directly received from said element, said image having a fixed edge, an optical unit for varying theposition of the other edge of said beam during movement of said beam, and a reflector for reflecting light from said element and said unit to said objective for forming a second image of light passing said element, said second image having a fixed edge and a movable edge reversed from that of said first image. -1

9. A sound recording optical system in accordance with claim 8 in which said reflector is so adjusted with respect to said element and said objective that the fixed edges of said images lie adjacent one another at a predetermined position from said obj ective. l

10. The method of changing a sound recording light beam having a single moving edge'to a light beam with two moving edges comprising directly varying one edge of a single light beam and reflecting light from said varying edge to produce a single beam having two Varying edges, one moving edge of said second mentioned single beam being produced by the directly varying edge of said first mentioned single beam and the other moving edge of said second mentioned single beam being produced by the reflected varying edge of said first mentioned single beam.

11. Apparatus for varying the exposure of a continuously moving photographic film, comprising means for producing in 'a plane parallel to said film, an illuminated rectangle, one edge of which is fixed and the other edge of which'is caused to move in accordance with the impulses to be recorded, optical means for imaging said rectangle on said film, and reflecting means for doubling the width of said image, said reflecting means comprising a mirror making an acute angle with the axis of said optical means, one

edge of said mirror being substantially adjacent to the fixed edge of said rectangle.

12. Apparatus for varying the exposure of a continuously moving photographic film, comprising means for producing in a plane parallel to said film, an illuminated rectangle, one edge of which is fixed and the other edge of which is caused to move in accordance with the impulses to be recorded, optical means for imaging said rectangle on said film, and reflecting means for 10 doubling the Width of said image, the fixed edge of said rectangle being in a plane at right angles to the direction of motion of said film.

EDWARD W. KELLOGG.

10 REFERENCES CITED The following references are of record in the file of th Number Number 

